Diaryl chlorophosphate finds extensive uses in the preparation of antibiotic products such as imipenem and in peptide synthesis, Fieser and Fieser, Reagents For Organic Synthesis, Wiley, 1967, p. 346.
Past methods of preparation of diaryl chlorophosphates have included reacting POCl.sub.3 with a stoichiometric amount of phenol. Even using a 1:1 mole ratio of POCl.sub.3 to phenol, three products are formed, i.e., phenyl dichlorophosphate, diphenyl monochlorophosphate and triphenyl phosphate, as well as unreacted POCl.sub.3, the first being in the predominant amount. While the POCl.sub.3 can be distilled off and recycled, the phenyl dichlorophosphate is difficulty separable and even when separated finds little utility in making the desired product. The triphenyl phosphate can be separated by distillation but is not usable in formation of the product. Phosphate as well as phenol values are, therefore, lost.
In U.S. Pat. No. 3,965,220, it has been proposed to prepare diaryl phosphorochloridates by first reacting phosphoryl chloride with a molar equivalent of phenol in the presence of an amine at a temperature ranging from about 85.degree. C. to about 135.degree. C. to prepare the aryl phosphorodichloridate followed by reacting that product with a molar equivalent of the same or different phenol at higher temperatures. While high yields of phosphorochloridate are claimed using this process, the reaction will still form triaryl phosphate, and other non-useful by-products.
In the preparation of alkyl iodides the following reaction scheme has been set forth: EQU (C.sub.6 H.sub.5).sub.2 P--OR+I.sub.2 .fwdarw.(C.sub.6 H.sub.5 O).sub.2 P(O)I+RI.
This is known in a paper entitled "The Reactions of Phenyl Esters of Phosphorus Acid with Iodine", JACS, 75 (1953) pp. 3145-3148. The starting reactant is prepared by reacting diphenyl chlorophosphite with cyclohexanol in the presence of pyridine and an ether solvent. By this means a cyclohexyl diphenyl phosphite is prepared. This product, when reacted with iodine, will give the alkyl iodide. This reaction has the same inherent problem as the previously discussed reaction. The diaryl chlorophosphite is prepared by reacting PCl.sub.3 with two moles of phenol. A mixture of mono-, di- and triphenyl phosphites is prepared which requires separation in order to obtain a pure starting material. As the molecular weights rise because of the weight of the aryl substituents, the difficulty in separating the products by distillation increases. This is particularly true when the aryl group is substituted such as in the case of a dihalo substituent.
Part of this problem is avoided in another method for converting alcohols into iodides by replacing the diphenyl chlorophosphite with o-phenylene chlorophosphite prepared from catechol. In this case the o-phenylene chlorophosphite is a stable reagent, prepared in a form which can be easily separated by distillation (see the paper entitled "A Useful Method for the Conversion of Alcohol Into Iodides", E. J. Corey et al., J. Org. Chem., 32 (1967) pp. 4160-4161).
It has now been found that alkyl diaryl phosphites and diaryl halophosphates can be easily prepared under conditions that reduce by-product formation and allow product recovery in high yields.